TheoA, I know eactly what you mean. I flew sailplane when I lived in Socal in the same club (SULA) that many of the F3B champs (Joe Wurts, Daryl Perkins) were in. It was amazing to watch them put the hammer down on the winch and stretch the line by flying side to side at the end of the launch before release to zoom up another 200-300 feet. No glow engine plane would have survived 3 seconds into a launch like that.

Anyway, to answer your question, no, spars are not always necessary. I'm sure you'll hear differing opnions on this, but typically, if you use full sheeting or full composite coverage on the wing, you can do without spars, or at most use balsa square stock for spars. If you vac bag, you have to use a solid core with no lightening holes anyways, so that'll be stronger still. If you do use fiber glass though, I would suggest running some widths of carbon tape under the FG on the thickest part of the wing to stiffen up the wing and add strength.

Ultimately, whether you should add wood spar or not really depends on the wing design. Good luck.